The role of viruses in marine phytoplankton mortality

Unicellular algae (phytoplankton) form the basis of the marine foodchain and thus play a critical role in the ocean’s geochemical processes. It is thus somewhat surprising that the mechanisms driving their mortality are still not fully understood. Traditionally, grazing by microzooplankton and sedimentation are considered the most important sources of phytoplankton loss. Other causes of phytoplankton death were more recently emphasized, including viral infection. Despite the awareness of high abundance and diversity of algal viruses, the extent and the factors regulating virally induced mortality remain understudied. This dissertation meaningfully enhanced the understanding of the quantitative significance of viruses as mortality agents for marine phytoplankton in environments with contrasting trophic status (nutrient-rich or eutrophic vs. nutrient-poor or oligotrophic). Viruses were, next to microzooplankton, responsible for the demise of the Phaeocystis globosa spring bloom thriving in the eutrophic southern North Sea. Interestingly, twelve viruses specifically infecting P. globosa harbored a high level of phenotypic diversity despite a close genetic relatedness. The impact of viruses on phytoplankton carbon production was higher in the eutrophic waters than in the oligotrophic environments. In these latter systems, virally induced algal mortality differed geographically, and more importantly, it varied for the different phytoplankton groups investigated. Our results suggest that environmental variables influence the magnitude of virally induced phytoplankton lysis. Laboratory studies highlighted that irradiance strongly and species-specifically affected virus-algal host interactions. The results strengthen the present call for more detailed studies on the role of viruses in the ocean’s functioning and the underlying mechanisms

Microbial food web dynamics in response to a Saharan dust event: results from a mesocosm study in the oligotrophic Mediterranean Sea

BiogeosciencesInternational audienceThe significant impact of dust deposition on het-erotrophic bacterial dynamics in the surface oligotrophic ocean has recently been evidenced. Considering the central role of bacteria in the microbial loop, it is likely that dust deposition also affects the structure and the functioning of the whole microbial food web. In the frame of the DUNE project, aiming to estimate the impact of dust deposition on the oligotrophic Mediterranean Sea through mesocosm ex-periments, the main goal of the present paper was to as-sess how two successive dust deposition events affect the dynamics of the microbial food web. The first dust seeding delivered new P and N to the amended mesocosms and re-sulted in a pronounced stimulation of bacterial respiration. It also induced pronounced, but transient, changes in the bac-terial community composition. No significant effects were observed on the abundances of viruses and heterotrophic nanoflagellates. The second dust seeding also delivered new P and N to the amended mesocosms, but the effect on the microbial food web was very different. Bacterial respira-tion remained constant and bacterial abundance decreased. Compositional changes following the second seeding were minor compared to the first one. The decrease in bacterial abundance coincided with an increase in virus abundance, resulting in higher virus : bacteria ratios throughout the sec-ond seeding period. Our study shows that dust deposition to the surface oligotrophic ocean may involve important mod-ifications of the trophic links among the components of the microbial food web with presumed consequences on C and nutrient cycling

Therapeutic Potential of a New Jumbo Phage That Infects Vibrio coralliilyticus, a Widespread Coral Pathogen

Biological control using bacteriophages is a promising approach for mitigating the devastating effects of coral diseases. Several phages that infect Vibrio coralliilyticus, a widespread coral pathogen, have been isolated, suggesting that this bacterium is permissive to viral infection and is, therefore, a suitable candidate for treatment by phage therapy. In this study, we combined functional and genomic approaches to evaluate the therapeutic potential of BONAISHI, a novel V. coralliilyticus phage, which was isolated from the coral reef in Van Phong Bay (Vietnam). BONAISHI appears to be strictly lytic for several pathogenic strains of V. coralliilyticus and remains infectious over a broad range of environmental conditions. This candidate has an unusually large dsDNA genome (303 kb), with no genes that encode known toxins or implicated in lysogeny control. We identified several proteins involved in host lysis, which may offer an interesting alternative to the use of whole bacteriophages for controlling V. coralliilyticus. A preliminary therapy test showed that adding BONAISHI to an infected culture of Symbiodinium sp. cells reduced the impact of V. coralliilyticus on Symbiodinium sp. photosynthetic activity. This study showed that BONAISHI is able to mitigate V. coralliilyticus infections, making it a good candidate for phage therapy for coral disease

First Viruses Infecting the Marine Diatom Guinardia delicatula

The marine diatom Guinardia delicatula is a cosmopolitan species that dominates seasonal blooms in the English Channel and the North Sea. Several eukaryotic parasites are known to induce the mortality of this species. Here, we report the isolation and characterization of the first viruses that infect G. delicatula. Viruses were isolated from the Western English Channel (SOMLIT-Astan station) during the late summer bloom decline of G. delicatula. A combination of laboratory approaches revealed that these lytic viruses (GdelRNAV) are small tailless particles of 35–38 nm in diameter that replicate in the host cytoplasm where both unordered particles and crystalline arrays are formed. GdelRNAV display a linear single-stranded RNA genome of ~9 kb, including two open reading frames encoding for replication and structural polyproteins. Phylogenetic relationships based on the RNA-dependent-RNA-polymerase gene marker showed that GdelRNAV are new members of the Bacillarnavirus, a monophyletic genus belonging to the order Picornavirales. GdelRNAV are specific to several strains of G. delicatula. They were rapidly and largely produced (<12 h, 9.34 × 104 virions per host cell). Our analysis points out the host's variable viral susceptibilities during the early exponential growth phase. Interestingly, we consistently failed to isolate viruses during spring and early summer while G. delicatula developed important blooms. While our study suggests that viruses do contribute to the decline of G. delicatula's late summer bloom, they may not be the primary mortality agents during the remaining blooms at SOMLIT-Astan. Future studies should focus on the relative contribution of the viral and eukaryotic pathogens to the control of Guinardia's blooms to understand the fate of these prominent organisms in marine systems

Population Dynamics and Diversity of Viruses, Bacteria and Phytoplankton in a Shallow Eutrophic Lake

We have studied the temporal variation in viral abundances and community assemblage in the eutrophic Lake Loosdrecht through epifluorescence microscopy and pulsed field gel electrophoresis (PFGE). The virioplankton community was a dynamic component of the aquatic community, with abundances ranging between 5.5 × 107 and 1.3 × 108 virus-like particles ml−1 and viral genome sizes ranging between 30 and 200 kb. Both viral abundances and community composition followed a distinct seasonal cycle, with high viral abundances observed during spring and summer. Due to the selective and parasitic nature of viral infection, it was expected that viral and host community dynamics would covary both in abundances and community composition. The temporal dynamics of the bacterial and cyanobacterial communities, as potential viral hosts, were studied in addition to a range of environmental parameters to relate these to viral community dynamics. Cyanobacterial and bacterial communities were studied applying epifluorescence microscopy, flow cytometry, and denaturing gradient gel electrophoresis (DGGE). Both bacterial and cyanobacterial communities followed a clear seasonal cycle. Contrary to expectations, viral abundances were neither correlated to abundances of the most dominant plankton groups in Lake Loosdrecht, the bacteria and the filamentous cyanobacteria, nor could we detect a correlation between the assemblage of viral and bacterial or cyanobacterial communities during the overall period. Only during short periods of strong fluctuations in microbial communities could we detect viral community assemblages to covary with cyanobacterial and bacterial communities. Methods with a higher specificity and resolution are probably needed to detect the more subtle virus–host interactions. Viral abundances did however relate to cyanobacterial community assemblage and showed a significant positive correlation to Chl-a as well as prochlorophytes, suggesting that a significant proportion of the viruses in Lake Loosdrecht may be phytoplankton and more specific cyanobacterial viruses. Temporal changes in bacterial abundances were significantly related to viral community assemblage, and vice versa, suggesting an interaction between viral and bacterial communities in Lake Loosdrecht

Diversité et importance écologique des virus dans le milieu marin

L’océan est le réservoir le plus important de virus sur la planète avec des abondances allant jusqu’à plusieurs milliards par litre. Ces virus sont une force directrice majeure pour l’évolution et la structuration du monde microbien, mais aussi pour le fonctionnement des grands cycles biogéochimiques dans les écosystèmes marins. Grâce aux techniques de séquençage à haut débit, nous commençons à entrevoir la diversité et la complexité de cette virosphère marine. Cette synthèse décrit les découvertes importantes dans le domaine de l’écologie virale marine et aborde la diversité de ces micro-organismes fascinants, leur impact sur la mortalité microbienne et les cycles de nutriments et d’énergie dans l’océan

Les virus marins. Simples parasites ou acteurs majeurs des écosystèmes aquatiques ?

Les virus n’en finissent pas de nous fasciner et de nous faire peur, du fait de leur caractère pathogène pour les plantes et les animaux. Dans les mers et les océans, ils sont des acteurs majeurs du fonctionnement des écosystèmes. Les virus constituent l’entité biologique la plus petite, la plus abondante et la plus diversifiée connue à ce jour en milieu marin, où ils exercent de multiples fonctions. Outre leur rôle dans la mortalité cellulaire et la redistribution de la matière, les virus sont aussi un puissant vecteur d’évolution de leurs hôtes et représentent une source majeure de diversité génétique, riche de découvertes pour la recherche fondamentale et appliquée. Cet ouvrage a pour but de mettre en lumière une communauté biologique invisible et essentielle pour comprendre et expliquer le fonctionnement des océans. Les auteurs, spécialistes de l’écologie et de l’évolution des virus aquatiques, montrent de quelle façon les virus régulent les écosystèmes microbiens, à la base des chaînes alimentaires des océans, et comment leur action pourrait, le cas échéant, agir sur le climat. Cet ouvrage est destiné aux étudiants en sciences de la vie et de la Terre, enseignants, chercheurs et universitaires, ainsi qu’au grand public intéressé par le sujet des virus et des océans

Diatom Viruses

International audienceThe discovery, isolation, and cultivation of the first diatom-infecting virus less than two decades ago revealed an enigmatic, ecological interaction that altered our understanding of diatom ecosystem functioning. Since that discovery, characterization of additional diatom host-virus systems has brought important insight into unique aspects of these viruses and the biogeochemical consequences of virus-mediated mortality. Emerging approaches for identifying these pathogens in natural populations are revealing widespread prevalence and geographic distribution of diatom viruses and the environmental factors that influence host-virus interactions. In this chapter, we summarize the existing literature and highlight the latest research on diatom viruses and the potential of these viruses to impact one of the most significant groups of phytoplankton on the planet. We conclude with thoughts for the future generation of diatom viral ecologists